Removing material from the terra firma, whether it be in reconstruction of highways, trenching operations or long wall and other mining operations, has seen numerous improvements in mechanisms to achieve such material removal in recent years. In order to lessen the down time of such material removal machinery, various improvements have been made to bit assemblies, which define the end point at which the machinery separates surface material from the underlayment or ground. This end point where the material removing equipment contacts the surface of the material to be removed is traditionally comprised of a series of bit assemblies that may include bits having a pointed forward end, bit holders in which the bits are mounted or could be made an integral part of, and bit holder blocks in which the base of the bit/bit holder is mounted. The bit holder block is mounted on either an endless chain or chain plate system or a rotatable drum.
Presently, the most common use of this bit assembly is found on the rotatable drum wherein numerous such assemblies are mounted, either in V-shape or spiral form on the drum. Such a recent improvement is found in U.S. Pat. Nos. 6,371,567 and 6,585,326 wherein the bit holder or middle piece of the bit assembly is no longer required to be retained on the bit holder block by a threaded shank with a nut thereon holding the bit holder on the bit holder block. This improvement includes a hollow shank on which the distal end is axially slotted and wherein the shank may be driven into a bore in the bit holder block and the distal end of the shank is compressed radially with a sufficient radial force between the bit holder shank and the bit holder block bore to maintain the bit holder mounted on the bit block during use.
Eliminating a retaining nut or retaining ring from the distal end of the bit holder shank eased the ability to remove the bit holder from the bit holder block through the bottom of the bit holder block. Further, a tungsten carbide tipped bit could be removed from the bit holder by punching same outwardly through the bottom of the bit holder block bore.
Another improvement in bit assemblies has been the introduction of diamond tipped bits or combination bit/holders. The hardened bit tips may be formed of man-made PCD material, or industrial powdered diamond material embedded in a core or base forming a coating on the tip of the bit/holder. With the introduction of this extremely hard material on the tip of the bit cutting assembly, the use of tungsten carbide bits mounted on bit holders which, in turn, are mounted on bit holder blocks, has in some instances given way to a unitary combination bit/bit holder which has a longer in use life than the prior tungsten carbide tipped three piece combination. It should also be noted that, if desired, a diamond tipped bit may also be utilized in conjunction with already existing bit holders and bit blocks.
In the case of tungsten carbide tipped bits, it may be preferred that the bit have the ability to rotate in the bit holder to spread out the wear characteristics of the bit during use. However, the longer use life of diamond tipped surface removal machinery means that the distal tip no longer has to be rotatable.
Another improvement in the material removing process has been not only the use of regular surface milling equipment which has the spiral mounted bit assemblies customarily positioned at ⅝ inch axially center-to-center in spiral or V-shape fashion across the drum, but also the use of micro-milling equipment wherein the bit tip spacing is 0.200 inch center line to center axial spacing between the bits. Micro milling is used not only to remove materials that regular milling achieves, but also to level parts of bumpy surfaces of roads, or remove just the upper portion of the road surface, perhaps an inch or two, to smooth the road surface, or to allow the delaying of resurfacing, thus achieving additional road surface life and saving money.
The use of many more bit assemblies on a single drum, sometimes utilizing about 900 such bit assemblies on a 46-54 inch diameter drum, means that the bit assemblies are mounted on the drum in much closer orientation to each other, thus minimizing the space between the bottom end of one bit holder block and the tip of an adjacent bit holder block. This decrease in adjacent space between bit blocks means that it is even more difficult than previously known to get access to the bottom of the bit holder block in order to drive out the bit holder, or any combination bit/holder from the bit holder block. Structures that increase the adjacent distance between the forward end of bit assemblies and the rear of adjacent bit assemblies provide more room for maintenance personnel to replace bits, holders, or combination bit/holders.